The History of Azulenyl Squaraines

Abstract

The synthesis of the first azulenyl squaraine dye was reported in 1966 and the paucity of activity in the scientific literature since that time has given the false impression that they have remained relatively unexplored as organic dyes. In contrast, a wealth of research activity on azulenyl squaraines has been recorded in the patent literature and it is the purpose of this review to bring these reports to the attention of scientists that have an interest in the chemistry and applications of squaraine dyes. This review shows that it is possible to prepare, via targeted substitution of the azulene ring, a dye with an absorption maximum value ranging from 650 to 840 nm.